There are two prep methods I've generally used with caustic bluing. One is to sand it off by hand, which is how the BAR went, and run it through the setup at work...that was an adventure. Once I got it completely dissembled the barreled action had about an inch of clearing either end of the tank. I did have to work out a couple of uneven spots that weren't apparent under the parked finish, but nothing too bad. Initially I toyed with the idea of rust bluing, but...there's a lot of steel on a BAR.

The other, which is a quicker and easier but sometimes unpredictable method is to blast a piece down to bare metal and work it over with a wire wheel before it goes in the salts. Although this worked well for me at school I haven't quite been able to duplicate the results at the new shop.

The latter is what I used on my Thompson and Model 12 projects and again in a later attempt to match Colt's WWI 'black army' finish.